1. Technical Field
The present invention relates to an optical module and an optical communications device used for optical communication.
2. Related Art
There are optical modules of various forms. One known example is an optical module of a structure in which a monitoring light receiving element receives part of light emitted from a light emitting element to monitor the quantity of the light. In this case, the quantity of the light emitted sometimes varies with the ambient temperature. Thus, related art technology has aimed at minimizing the temperature characteristic of each member, such as a monitoring light receiving element or a lens, in order that a proportionality relation may be maintained between a monitored quantity of light and a fiber coupled quantity of light, regardless of the ambient temperature.
JP-A-2004-72072 and JP-A-10-65189 are examples of related art.
Furthermore, JP-A-8-236807 discloses a technique that uses a multilayer transparent film (27) having transmissivity that shows a low value at a wavelength, at which the emission intensity is high, of light emitted from a luminous layer (24), thereby reducing temperature-caused variation in the output of light.
The inventors has examined a way to keep the quantity of light emitted from a light emitting element constant by feedback controlling the driving current off the light emitting element on the basis of the quantity of light received by an optical module (a module for optical communication). In the case where a vertical cavity surface emitting laser (VCSEL) is used as the light emitting element, feed back control (Auto Power Control (APC)) is particularly important to keep the quantity of light constant because, in this case, significant variation occurs in the quantity of light with changes in the ambient temperature, decreasing the quantity of the emitted light as the temperature rises.
In the case of an optical module with a band of 850 nm or more, on the other hand, it is necessary to increase the driving current in order to permit a high speed drive even at a low temperature. This is because of a phenomenon that the drive bandwidth of an element decreases where the driving current diminishes due to a low temperature. The output of light increases in this case. Therefore, a partial reflection film is used to attenuate the quantity of light (attenuation) and secure the safety of human eyes, and other things.
However, if the driving current of the above described light emitting element is feedback controlled based on the attenuated light, the driving current will become even larger at a high temperature. As a result, the driving current may transcend its limit value (the upper limit), becoming unable to operate, because of a shortage of output, at a temperature of a certain degree or higher. In contrast, if the element is operated with a constant driving current, the operating temperature limit may become wider, but variation in the quantity of light will increase, as described above.
In addition, in the case of a laser beam, there is a value of current for each degree of temperature, which value permits the laser beam to emit light most efficiently. Therefore, use of a driving current of such value will make the operating temperature limit wider and, furthermore, improve the transmission characteristics including, for examples the speed of operation and reduction of noise.